systemc: Also look for tests in the tlm test directory.

systemc: Also look for tests in the tlm test directory.

Both basic systemc and tlm tests were present, but scons only looked
in the systemc directory when populating the test json. This change
makes it also look in the tlm directory so that those tests can be run.

Change-Id: Id65b744664350f6105fb3a4f28cbc7ab91d8c82e
Reviewed-on: https://gem5-review.googlesource.com/c/15056
Reviewed-by: Anthony Gutierrez <anthony.gutierrez@amd.com>
Maintainer: Anthony Gutierrez <anthony.gutierrez@amd.com>